The circumference of a circle is equal to the sum of the circumference of two circles having diameters 36 cm and 20 cm.

(b) 28 cm
Let r cm be the radius of the new circle.
We know:
Circumference of the new circle = Circumference of the circle with diameter 36 cm + Circumference of the circle with diameter 20 cm
Thus, we have:

$2 \pi r=2 \pi r_{1}+2 \pi r_{2}$

$\Rightarrow 2 \pi r=(2 \pi \times 18)+(2 \pi \times 10)$

$\Rightarrow 2 \pi r=2 \pi \times(18+10)$

$\Rightarrow 2 \pi r=(2 \pi \times 28)$

$\Rightarrow 2 \pi r=\left(2 \times \frac{22}{7} \times 28\right)$

$\Rightarrow 2 \pi r=176$

$\Rightarrow 2 \times \frac{22}{7} \times r=176$

$\Rightarrow r=\left(176 \times \frac{7}{44}\right)$

$\Rightarrow r=28 \mathrm{~cm}$